A photon of frequency f, and another of frequency f' (take f' as given) collide to create an electron-positron pair. The frequency f is such that when the collision is head on, there is exactly enough energy to produce the electron-positron pair at rest in the center-of-mass frame. What is f)
So in the rest frame, $E=h(f+f'), p=h\frac{f-f'}{c}$. We know that in the center-of-mass frame, $E'=2m_ec^2$. So my idea was to find the velocity of the center of mass frame, and then $E=\gamma E'$, but this seems overly complicated, is there an easier way?